I. Hargittai

Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G*(6D) basis set. Least-squares refinement of a model withCs symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, α=119.0±0.2‡ (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the π system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1‡, 120.7±0.1‡, and 119.0±0.3‡, respectively. Important bond distances are 〈rg(C-C)〉=1.398±0.003 å andrg(C-N) =1.407±0.003 å. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is 〈¦Τ¦〉=44±4‡. The equilibrium dihedral angle is calculated to be 41.8‡ at the HF level and 43.6‡ at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(Cortho-Cmeta) -r(Cipso-Cortho) is 0.008–0.009 å. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 å at the HF level and 0.072 å at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.

Combined electron diffraction/mass-spectrometric investigation of the molecular structure of germanium dichloride

Abstract Germanium dichloride was produced by reaction of Ge + GeCl 4 at 660°C in a combined electron diffraction/quadrupole mass-spectrometric experiment. The structure of germanium dichloride can be characterized by the following parameters: r a (GeCl): 2.183 (4) A, l (GeCl): 0.080 (2) A, ∠ClGeCl: 100.3(4)°, l (ClċCl): 0.166 (4) A. The geometrical variations observed in Group IV dihalides are consistent with the VSEPR model, and with considerations based on non-bonded interactions.

The influence of torsional vibrations on the molecular configuration determined by gas electron diffraction

Abstract As a consequence of intramolecular vibrations distorted apparent structures may result from an electron diffraction analysis of molecules possessing symmetrical equilibrium configuration. The amount of torsional distortion gives information concerning the barrier height to internal rotation. An approach is suggested to estimate barrier heights on the basis of average torsional angles as determined from electron diffraction, and expressions of the rotation-dependent distances as obtained from a Taylor expansion by neglecting higher order terms.

The molecular structure of tetramethoxysilane in the gas phase, an electron diffraction study

Abstract The electron diffraction study of tetramethoxymethane showed that in the gas phase the molecule has S 4 symmetry, flattened along the S 4 axis. Central and peripheral C-O bond lengths are different, consistent with considerations based on the anomeric effect. Comparison is made with ab initio calculations on methanediol. The geometrical parameters ( r g (1) structure) are: central C-O bond 1.395 A; peripheral C-O bond 1.422 A; C-H bond 1.11 A; O-C-O angle bisected by the S 4 . axis 114.7°; C-O-C angle 114.0°; O-C-H angle 111.9°; methoxy torsional angle 63.1°; methyl torsional angle 48.5°.

Electron diffraction study of the molecular structure of germanium dibromide

Abstract The molecular structure of ground-state monomeric germanium dibromide (rg 2.337 ± 0.013 A, ∠101.2 ± 0.9°) has been determined by electron diffraction. The GeBr2 is produced by a reaction between Ge metal and GeBr4 vapour. Experimental data may indicate the presence of another state.

Spectroscopic studies of the AlCl3 · NH3 complex

Abstract A normal coordinate analysis for the AlCl 3 · NH 3 complex is performed. Calculations for a hypothetical complex with a planar AlCl 3 ligand are included. A deviation from planarity is found in the realistic complex. Force fields for free NH 3 and AlCl 3 molecules are employed in the construction of an initial approximate force field. The theory of kinematic coupling is used to study the frequency shifts from free to complexed ligands. A final force field is developed so as to reproduce exactly a set of observed frequencies from the literature. Calculations on mean amplitudes and perpendicular amplitude correction coefficients are reported.

Two independent gas electron diffraction investigations of the structure of plumbous chloride

Abstract The results of two independent electron diffraction analyses of PbCl 2 are compared. The bond lengths ( r g ) and angles ( r α ) were found to be 2.447 ± 0.005 A and 98.7 ± 1.0° (nozzle temperature 853 K, Budapest), and 2.444 ± 0.005 A and 98.0 ± 1.4° (nozzle temperature 963 K, Moscow), respectively.

The molecular geometry of the addition compound Cl3Ga.NH3 as studied by electron diffraction

Abstract The molecular geometry of the complex of gallium trichloride with ammonia, Cl3Ga.NH3, has been studied by electron diffraction. The most important internuclear distances in terms of ra parameters are as follows: r(Ga-Cl) = 2.142±0.005A, r(Ga-N) = 2.057±0.011A, r(Cl⋯Cl) = 3.642±0.010Aand r(Cl⋯N) = 3.242±0.012A. As in the case of the aluminium analogue, the flat pyramidal configuration of the GaCl3 part of the complex suggests a planar equilibrium structure for free GaCl3. The distance between the donor and acceptor parts may indicate a somewhat weaker interaction than is the case in the aluminium analogue.

Molecular structure of trans- and cis-methylchlorovinyl sulphone

Abstract An electron diffraction analysis of trans and cis isomers of methylchlorovinyl sulphone is described. The bond lengths ( r g ) and bond angles with estimated total errors in parentheses in units of the last digit are as follows: trans isomer SO 1.437(3), SC mean 1.769(9), CCl 1.721(7), CH mean 1.136(6) A, OSO 119.5(8), OSC vinyl 109.2(6), SCC 117.8(7), CSC 102.4(12), CCCl 123.0(13), CCH 121(2), SCH 109.1(10)°; cis isomer: SO 1.437(3), SC mean 1.766(10), CH mean 1.116(5) A, OSO 120.3(10), OSC vinyl 111.1(7), SCC 127.6(10), CSC 101.2(15), CCCl 124.3(10), CCH 120(2), SCH 108(2)°. Two conformers are detected for each geometrical isomer. The CC bond tends to eclipse other bonds in the trans isomer. The absence of eclipsed conformers in the cis isomer may be ascribed to steric hindrance. This is consistent with the opening of the SCC, CCCl and OSC vinyl angles in this isomer as compared with the trans isomer.

Electron diffraction and vibrational spectroscopic investigation of the molecular structure of (chloromethyl)trichlorosilane

Abstract An electron diffraction analysis of the molecular structure of the title compound has been carried out, and related vibrational spectroscopic measurements and calculations have been made. The main bond lengths ( r g and bond angles r α ) are as follows: SiCl, 202.8(2); SiC, 185.1(10); CCl, 179.4(11); CH, 111.2(18) pm; SiCCl, 111.7(4);l ClSiC, 109.95(21)°. The conformation of the molecule is staggered. The barrier to internal rotation is estimated to be around 10 kJ mol −1 .